The present invention relates to an ultrasonic wave diagnostic apparatus for concurrently display an ultrasonic wave echo signal and a reference signal on a display apparatus such as a cathode ray tube.
A system for concurrently displaying an ultrasonic wave echo signal and a reference signal, for example, an electrocardiogram, on an identical CRT (cathode ray tube) is known as an M-mode display system. A PPM (pulse-position-modulation) method is known for displaying the reference signal on the CRT. In this method, the reference signal is pulse-position-modulated by a sawtooth wave, for example, to produce PPM (pulse-position-modulated) pulses which represent portions of the reference signal. The PPM pulses and an ultrasonic echo signal representing echo pulses are added and applied, as added, to the CRT as a signal to be modulated in intensity.
In the conventional systems, apparatus generates one sawtooth wave for the reference signal during the period of each rate pulse. Accordingly, if it is desired to shorten the sampling period of the reference signal and therefore obtain increased sensitivity, the frequency of the rate pulses must be increased. However, if the rate pulse frequency is increased, the diagnostic depth must be shallowed. Here, the diagnostic depth means the distance from the position of the ultrasonic wave transducer to an object to be diagnosed. Thus, if the frequency of the rate pulses is decreased in order to make the diagnostic depth deep, the sampling period of the reference signal is lengthened, making the accurate display of the reference signal on the CRT impossible.
A one conventional method utilized to overcome that disadvantage has been to display the reference signal with high frequencies such as a phonocardiogram, separately from the M-mode signal (the detected echo signals which vary with time to represent the movement of a part of the object being diagnosed). In such applications, a CRT of dual beam type is used. This conventional method, however, is impractical because of expensiveness of the CRT used.
When the echo signal reflected from the diagnostic object is sweeped in Y direction on the CRT to form a M-mode image, the sweeping rate of the CRT in the X direction is very slow, i.e. 1 to 10 seconds per one frame, and thus the intervals of the reference signal in the X direction are short. Therefore, the display of the reference signal in the X direction is not decisively important. Accordingly, improvement of only the Y directional sweeping, or the sweeping of the ultrasonic echo signal, suffices for practical needs.
Accordingly, an object of the present invention is to provide an ultrasonic wave diagnostic apparatus in which the display characteristic of the reference signal in the sweeping direction of the ultrasonic echo signal is improved by sampling a plurality of times the reference signal during the period of the rate pulses.